1. Field of the Invention
The present invention relates to a nanowire patterning method, and more particularly, to a method for growing and patterning nanowires on a substrate using a novel sacrificial material layer.
2. Related Art
A high-temperature growth process is required to synthesize semiconductor nanowires. Some soft nanowires cause etching difficulty during the fabrication of semiconductor devices. Hence, nanowire patterning has been positioning itself as an important technique.
In microelectronics technology, a lift-off process is a patterning process that produces a target material structure on the surface of a substrate using a sacrificial layer. This process is an additive technique unlike a traditional subtracting technique such as etching. The size of the structure produced in this process can be in the range from nanometer to centimeter, but is typically a micrometer scale.
An inverse pattern is firstly formed on a sacrificial stencil layer, which is deposited on the surface of a substrate. This patterning is carried out by etching the sacrificial stencil layer to form holes, and then depositing target material so as to reach the surface of the substrate through the holes, thereby forming a final pattern. The target material is deposited on the entire area of the substrate. Thus, the target material not only reaches the surface of the substrate through the etched areas, i.e. the holes, but also remains on top of the sacrificial stencil layer that is not etched. When the sacrificial stencil layer is removed by cleaning with a solvent, the target material atop the sacrificial stencil layer is lifted off and is cleaned off along with the underlying sacrificial layer. After the lifting off, the target material remains only on the area that is in direct contact with the substrate.
Recently, nanostructures are under intensive study regarding applications as a building block of various nanoscale devices. A great number of studies have been focused on the synthesis of nanostructures, which makes it possible to control geometry, aspect ratio (width-to-height ratio), growth rate and growth direction. Particularly, a high-temperature process absorbs attention since high-temperature environment has a variety of merits such as simple precise control of doping and thickness and the realization of high quality nanowires.
For future-oriented application of nanodevices, selective-area growth and/or post-growth patterning are becoming a big issue. An attempt is made at the selective-area growth on a patterned metal layer. Unfortunately, this attempt limits substrates or electrodes that can be selected for a specific application.
Meanwhile, in the microelectronic industry, a sacrificial layer and/or a lift-off technique have been used for patterning a functional material. Here, a lift-off material (e.g., photoresist) used up to now is not suitable, because a high-temperature growth process is required for the synthesis of nanowires. In detail, such a lift-off material cannot withstand the heat of the high-temperature growth process, and thus burns by the heat.
Meanwhile, in the case where a known sacrificial layer material such as porous silicon, polysilicon, silicon dioxide and polysilicate glass (PSG) is used, a toxic hazardous chemical such as HF has to be used to remove the sacrificial layer. However, the toxic chemical attacks and damages the nanowires. In some cases a ZnO film is also used as a sacrificial layer for the fabrication of microstructures due to its higher etch rate in both acid and alkali chemicals. Therefore, diluted chemicals have a possibility of attacking the nanowires and changing the geometry and size of the nanowires in an active area during a fabrication process.
As such, the use of a suitable soft lift-off/sacrificial layer is becoming a big concern in the fabrication of nano-electronics that use the nanowires as a functional material. While a cheap and soft sacrificial material is required and has stability under high temperature, there is not a sacrificial layer material, particularly a photoresist material, which can meet such requirements and simultaneously be used for stable growth and patterning of the nanowires. Therefore, according to the circumstances of the current technologies, there are still a number of difficulties against the application of nanowires.